Image enlargement processing is one of the most typical image processing techniques and various techniques have been proposed for image enlargement processing. In addition, it is well known in the art that edges are often blurred in an enlarged image obtained by image enlargement processing and various edge enhancement techniques for enlarged images have been also proposed.
For example, International Publication No. WO 2012/114574 A1 discloses an image enlargement method in which the pixel value of each pixel of a high-resolution image are calculated by interpolation of pixel values of multiple pixels of a low-resolution image. In this image enlargement method, appropriate interpolation processing is achieved by giving a large interpolation coefficient to a highly-correlated pixel of the low-resolution image independently of the shape of an edge, the highly-correlated pixel being highly correlated with a pixel of interest.
Japanese Patent Application Publication No. 2013-165476 A discloses a technique for improving the image quality, which involves generating an edge-enhanced enlarged image and correcting the enlarged image on the basis of the difference between the enlarged image and the input image.
Japanese Patent Application Publication No. 2011-049868 A discloses a technique for achieving edge enhancement, which involves extracting high frequency components of an input image and feeding the high frequency components back to an enlarged image.
Japanese Patent Application Publication No. 2009-094862 A discloses a technique which involves detecting the direction of an edge by applying a differentiation operator to an input image, calculating a candidate value as a weighted average based on the direction of the detected edge and calculating an interpolated value of an interpolation pixel by performing limiting processing on the candidate value.
Japanese Patent Application Publication No. 2007-193397 A discloses a technique for performing image scaling processing and edge enhancement at the same time by switching filter coefficients of a polyphase filter used for the scaling processing between a basic process portion and a high-frequency enhanced portion.
Japanese Patent Application Publication No. 2000-99713 A discloses a technique for obtaining an enlarged image by dividing a multi-value image into an edge portion and a non-edge portion, individually performing image enlargement processing on the edge portion and the non-edge portion and finally synthesizing the resultant enlarged images obtained from the edge portion and the non-edge portion.
Japanese Patent Application Publication No. H09-102867 A discloses a technique which involves analyzing an input image in units of windows each consisting of 2×2 pixels, and generating an enlarged image on the basis of the analysis.
Japanese Patent Application Publication No. H10-63826 A discloses a technique for generating a multi-value image data scaled by a desired scaling factor through performing image enlargement processing on a binary image with an enlargement factor of M and then performing an image reduction with a reduction factor of N.
Japanese Patent Application Publication No. 2001-94765 A discloses a technique which involves generating an enlarged image data by performing image enlargement processing on input image data, generating quality-improved image data by applying a filter to the enlarged image data and obtaining output image data by performing image reduction processing on the quality-improved image data.